The circuit in Fig.1 shows a simple method for varying the trigger angle and therefore, the power in the load. Instead of using a gate pulse to trigger the SCR,the gate current is supplied by an a.c. source of voltage $e_{s}$ through $R_{\text { min }}, R_{v}$ , and the series diode $D .$

The circuit operates as follows:

(a) As $e_{s}$ goes positive, the SCR becomes forward-biased from anode to cathode; however, it will not conduct $\left(e_{L}=0\right)$ until its gate current exceeds $I_{g(\min )}$ .

(b) The positive $e_{s}$ also forward biases the diode and the SCRs gate- cathode junction; this causes flow of a gate current $i_{g} .$

(c) The gate current will increase as $e_{s}$ increases towards its peak value. When $i_{g}$ reaches a value equal to $I_{g(\min )},$ the SCR turns "on" and $e_{L}$ will approximately equal $e_{s}$

(d) The SCR remains "on" and $e_L\approx e_{s}$ until $e_{s}$ decreases to the point where the load current is below the $\mathrm{SCR}$ holding-current. This usually occurs very close to the point until $e_{s}=0$ and begins to go negative.

(e) The SCR now turns off and remains off while $e_{s}$ goes negative since its anode-cathode is reverse biased, and since the SCR is now an open switch, the load voltage is zero during this period.

(f) The purpose of the diode in the gate-circuit is to prevent the gate-cathode reverse bias from exceeding peak reverse gate voltage during the negative half-cycle of $e_{s} .$ The diode is chosen to have peak reverse- voltage rating greater than the input voltage $E_{\text { max }} .$

(g) The same sequence is repeated when $e_{s}$ again goes positive.

The load-voltage waveform in Fig.1 can be controlled by varying $R_{v}$ which varies the resistance in the gate circuit. If $R_{v}$ is increased, the gate current will reach its trigger value $I_{g(\min } )$ at a greater value of $e_{s}$ making the SCR to trigger at a latter point in the $e_{s}$ positive half-cycle. Thus, the trigger angle $\alpha$ will increase.

The opposite will occur if $R_{v}$ is decreased. Of course, if $R_{v}$ is made large enough the SCR gate current will never reach $I_{g(\min )}$ and the $S C R$ will remain off. The minimum trigger angle is obtained with $R_{v}$ equal to zero.

As shown in Fig.1 the limiting resistor $R_{(\min )}$ is placed between anode and gate so that the peak gate current of the thyristor $I_{g m}$ is not exceeded. In the worst case, that is when the supply voltage has reached its peak, $E_{\max },$

$$R_{\min } \geq \frac{E_{\max }}{I_{g m}}$$

The stabilising resistor $R_{b}$ should have such a value that the maximum voltage drop across it does not exceed maximum possible gate voltage $V_{g(\max )}$ . From the voltage distribution,

$$R_{b} \leq \frac{\left(R_{v}+R_{\min }\right) \cdot V_{g(\max )}}{\left(E_{\max }-V_{g(\max )}\right)}$$

The thyristor will trigger when the instantaneous anode voltage, $e_{s},$ is

$$e_{s}=I_{g(\min )}\left(R_{v}+R_{\min }\right)+V_{d}+V_{g(\min )}$$

The resistance trigger shown in Fig.1 is the simplest and most economical circuit. However, it suffers from several disadvantages. First, the trigger angle $\alpha$ is greatly dependent on the SCR's $I_{g(\min )},$ which, as we known, can vary widely even among SCRs of a given type and is also highly temperature dependent. In addition, the trigger angle can be varied only up to an approximate value of $90^{\circ}$ with this circuit. This is because $e_{s}$ is maximum at its $90^{\circ}$ point and the gate current has to reach $I_{g(\min )}$ somewhere between $0-90^{\circ},$ if it will if at all. This limitation means that the load voltage waveform can only be varied from $\alpha=0^{\circ}$ to $\alpha=90^{\circ} .$